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The G-8, eight Northern Hemisphere industrialized countries, last month produced its first firm target for curbing rising global temperatures: no more than 2 degrees Celsius, 3.6 Fahrenheit, above pre-industrial levels.

World headlines trumpeted the target. Long the maximum ceiling acceptable to many scientists and environmental advocates, “2 degrees” has now been semi-enshrined as the consensus “magic number” for avoiding dangerous climate change.

Through 2008, the global average temperature had already warmed roughly 0.7 degrees Celsius, above preindustrial levels, meaning that by some measures we’re already one third of the way toward hitting the 2 degree ceiling. And temperatures are now rising faster than in earlier decades in the 20th century. (For more on the nuances of such measurements, see The Yale Forum‘s post last year.)

In the Fahrenheit measurements familiar to citizens of the U.S., the G-8 target means not allowing a global temperature average above roughly 61.6 degrees Fahrenheit.

The idea of this two-degree limit Celsius has a long scientific history stretching back to the 1980s. It started to be cemented in policy circles after the European Union adopted the target in 1996.

But the meaning of that simple number, many observers say, is more slippery than it appears. And though it’s supported strongly by Intergovernmental Panel on Climate Change data, it still involves some subjective analysis.

A Delicate Compromise … and a Value Judgment

IPCC chairman Rajendra Pachauri has said that “this whole issue of 2 degrees versus 1 degree or 1.5 degree is something based on a value judgment that essentially relates to what is dangerous … in terms of making it almost impossible for some people on this planet not being able to live in those locations.”

In other words, the temperature target may be too high for some people in the developing world who are already vulnerable. The IPCC’s 2007 report stated that even 2 degrees Celsius above pre-industrial levels is likely to have serious impacts, especially in terms of stressing water supplies and creating more malnutrition, disease, and drought.

2 degrees C = 3.6 degrees F

The Real Climate scientist bloggers strongly articulate that cautionary view: “We feel compelled to note that even a ‘moderate’ warming of 2 degrees C stands a strong chance of provoking drought and storm responses that could challenge civilized society, leading potentially to the conflict and suffering that go with failed states and mass migrations.”

MIT climate scientist Kerry Emanuel said in an e-mail exchange with The Yale Forum that 2 degrees is a “delicate compromise between what is desirable and what may be feasible.” He said there is bound to be debate on such a big issue.

“Any number much above 1 degree involves a gamble, and the odds become less and less favorable as the temperature goes up,” Emanuel wrote. “I do not think there is any ‘magic number’ that denotes some kind of tipping point, but if there is, we collectively have no idea what number that is.”

The tipping point idea has been debated in the media and in scientific circles, with no precise number or scenario agreed upon. But there are real worries.

James McCarthy, professor of biological oceanography at Harvard University and the current President of the American Association for the Advancement of Science, said that the closer the planet comes to a global average rise of 2 degrees Celsius, the less likely we are to be able to avoid climbing to even higher temperatures.

“As we approach 2 degrees, many models suggest that tipping points will be reached with respect to summer sea ice and these large masses of glacial ice,” McCarthy told The Yale Forum in an e-mail interview. “Either will result in strong reinforcing (positive) climate feedbacks, not to mention dramatic changes in the Arctic ecosystem and enhanced sea level rise.”

Models also suggest that feedback cycles could afflict both the tundra – where scientists fear a massive carbon release – and the Amazon forest ecosystem, which could rapidly dry up, McCarthy noted.

Temperature Targets a Distraction?

Some observers point out that the debate over 2 degrees involves two interrelated questions that are often blurred. First, how does the temperature goal stand up scientifically – i.e., would it still prevent perilous ocean level rise, large-scale drought, loss of ecosystems, dangerous weather, etc.?

Second, how does the target operate as a policy tool and as a piece of rhetoric in global negotiations?

That second question may now be the more relevant one in the run-up to this December’s Copenhagen negotiations. When asked about whether “2 degrees” is good or bad, some leading scientists wave away the question entirely. (Many scientists, moreover, think that 2 degrees is an improbable goal and that that ceiling likely will be exceeded.

“I think that arguments over temperature targets are a distraction,” Ken Caldeira, a climate scientist at Stanford University and the Carnegie Institution, wrote in an e-mail exchange with The Yale Forum.

“We should be talking about emissions targets, and the right emission target is zero,” Caldeira said. “We are going to solve the carbon-climate problem when we create an understanding that it is no longer acceptable to use the atmosphere as a waste dump … This should not be a discussion about targets, it should be a discussion about which kinds of objects people should be allowed to build.”

There is indeed a vigorous debate between the developing world and the G-8 over this very question.

Chief among the early critics of the G-8 effort was IPCC chairman Pachauri, who praised the “aspirational goal” on temperature but criticized the G-8 for not spelling out what sorts of specific, deep cuts in emissions member countries were willing to take.

In other words, it’s the emissions, not the temperature, that’s important.

Moreover, for developing countries, the 2 degrees Celsius target has the ring of evasion, according to Adil Najam, a lead author of the 2007 IPCC report and director of Boston University’s Pardee Center for the Study of the Longer-Range Future. He will be part of Pakistan’s delegation at Copenhagen in December.

In a telephone interview with The Yale Forum, Najam said that industrialized countries tend to talk about “common” goals in terms of reducing emissions. By contrast, developing countries focus on “differentiated” responsibilities – namely the need for the big industrial nations to take the deeper emissions cuts.

Najam said developing countries see the 2 degrees goal as a vague notion that does not put pressure on individual countries, since no one nation can control global temperature. “A lot of developing countries get very antsy about such targets because they do not contain any responsibility,” he said.